1. Field of the Invention
This invention relates to methods and apparatus to mix thermally stratified potable water supplies to prevent freezing.
2. Description of the Related Art
When a large capacity tank is underutilized, differential thermal conditions in the tank can cause the contents to stratify in thermoclines, where warmer layers of water meet cooler layers. If, as is often the case, a tank with stratified contents is both filled and emptied from a limited portion of the tank, water supplied by the tank will be from recently filled, fresher strata, while the remaining strata in the tank may age, unused and relatively undisturbed. For non-insulated or under-insulated tanks in colder regions during winter months, the aging, unused strata of water in such tanks may lose sufficient heat to freeze. When ice forms in a water storage tank, the effective liquid capacity of the tank is reduced by the volume of ice in the tank. If a sufficient volume of ice is formed, the effective liquid capacity of the tank may be reduced by such an amount that it is not sufficient for water supply needs.
Managers of water supply systems, such as municipalities, have employed various means to minimize formation of ice in water supply tanks, with varying degrees of success. In some systems, heating is used to prevent ice formation. In some such systems, water is heated. In some cases, liquid water from the tank is pumped and circulated through a heat exchanger to raise its temperature. In other cases, at least some of inflowing water is heated during filling of the tank. In yet other cases, steam is injected into the liquid water in the tank to raise its overall temperature. Such water tank heating systems are expensive and require considerable maintenance.
In other systems, small sparging bubbles are provided to water in lower portions of the tank, exchanging heat from the air trapped in the bubbles to the water surrounding them as the bubbles rise through the tank. If the air provided to form the bubbles is significantly warmer than the water through which the bubbles pass, and if a sufficient quantity of air is bubbled through the tank, such sparging can elevate the temperature of some of the liquid water and somewhat reduce ice formation. Because the heat capacity of air is relatively small, however, to be effective such systems must provide a very large volume of sparging bubbles and, preferably, the air forming the bubbles must be heated to a considerably high temperature. For tanks with significant ice formation tendencies, such systems are either ineffective or very expensive.
What is needed is a method of preventing or remediating stratification of water in storage tanks to preclude ice formation in the first place. As will be understood by those in the art, stratification can be obviated by sufficient vertical mixing of water in the tank, mixing warmer water from recent fillings with cooler water from prior fillings. Such mixing can also assist in melting ice already formed from water previously thermally stratified in the tank. The effectiveness of such mixing for ice remediation may be enhanced by providing heat to water that is to be mixed.
A number of means for mixing liquids are available to de-stratify stored water. A mechanical mixer, comprised of a screw or blade that is turned by a motor, is commonly employed to mix various liquids. Mechanical mixers, however, are subject to a number of shortcomings for mixing drinking water in storage tanks.
Mixing the strata in a typical large water storage tank with a mechanical mixer requires a large amount of energy relative to the amount of water that is actually mixed. Further, agitation of the water in the tank by mechanical mixers can disturb sediment settled in the bottom of the tank, resulting in suspended sediment degrading the aesthetics of the water for drinking. Further still, mechanical mixers are often inefficient, mixing some but not all strata in a storage tank. In addition, acquisition costs can be high for a mechanical mixer having sufficient capacity to mix all the strata in a large storage tank. Yet further, costs are high to retrofit an existing water storage tank with a mechanical mixer, retrofitting further often entailing a need to drain the tank or otherwise temporarily remove the tank from the water distribution system. What is needed are more economical and efficient means of mixing water to eliminate stratification with minimal disturbance to sediment in the tank. What is needed further is such means that can be retrofitted to a water storage tank operation economically and without a need to take the water tank off-line.
For economy, it is further desirable that the mixer that is used to obviate stratification be engaged only when needed, i.e. only when thermal stratification is taking place and cooler strata are at risk of freezing. Accordingly, it is desirable to have a means for determining when mixing is needed and for engaging the mixer only at such times.
It is further desirable that the mixer system be easy to install and easy to operate.